Silica glass has been heavily utilized in the optical, semiconductor, electronic, and chemical industries due to its excellent heat resistance, corrosion resistance, and optical properties.
Historically, the silica glass has been produced by melting natural quartz in an electric furnace or with an oxyhydrogen flame. Alternatively, silica glass can be made by subjecting silicon tetrachloride to high-temperature oxidation and melting in an oxyhydrogen flame or a plasma flame. These techniques, however, are expensive, because they require temperatures of 2000.degree. C. or higher. To withstand such high temperatures and form a quality product, durable starting materials must be utilized.
More recently, silica glass has been synthesized at low temperatures using the sol-gel process. In accordance with this technique, water is added to a silicon alkoxide represented by the formula: EQU Si(OR).sub.4
where
R is an alkyl group, and/or a silicon alkoxide polycondensate represented by the formula: EQU (RO).sub.3 Si-[OSi(OR).sub.2 ].sub.4 -OSi(OR).sub.3 PA1 R is an alkyl group and n is zero or an integer of 1 to 8. PA1 R is hydrocarbon group, and PA1 n is 4 PA1 M is a metal alkoxide, PA1 R is a hydrocarbon radical, and PA1 n is generally the valence of M. Generally, silicon alkoxide is used at a level to produce a final glass product with a silicon dioxide level of 40-100 weight percent, preferably 100 weight percent.
where
This aqueous silicon alkoxide or alkoxide polycondensate is then hydrolyzed to form a silica sol by adding an alcohol to produce a uniform system of the silicon alkoxide, water, and alcohol. The silica sol is then allowed to stand and form a gel which is dried and sintered in a suitable atmosphere to form a silica glass.
Despite the advantages of the sol-gel process, there are still problems associated with it. For example, the presence of --Si--OH groups at the surface of the gels, made in accordance with this process, can cause bloating and foaming of the gel during the final stages of sintering. Generally, some dehydroxylation takes place during the early stages of sintering according to the following reaction: EQU --Si--OH+--Si--OH.fwdarw.--Si--O--Si--+H.sub.2 O
Even with such dehydroxylation, the final sintered glass still has as much as 2000-4000 ppm of --Si--OH groups. Moreover, subsequent heating of the glass product to its softening point, during fiber drawing or sealing, may cause bloating. High hydroxyl levels are also detrimental to optical communication uses where infrared radiation transmission is vital. The need for dehydroxylation is extensively discussed in C. J. Brinker et al., Sol-Gel Science-The Physical and Chemistry of Sol-Gel Processing, pp. 628-72 (1990).
One approach to the hydroxyl problem has been the treatment of the gel, during sintering, with chlorine gas at 800.degree. C. and then with oxygen at 1000.degree.-1100.degree. C. See K. Susa et al., "Reduction of Chlorine Content in Sol-Gel Derived Silica Glass," Journal of Non-Crystalline Solids, vol. 79, pp. 165-76 (1986). This technique, however, complicates the sintering step and requires specialized handling systems for the chlorine gas.
Dehydroxylation has also been desired for silica glass derived from non-sol-gel techniques. For example, fully dense alkali borosilicate glass, which has been made porous by heating and then acid leaching, has been treated with a variety of dehydroxylation agents. Such glass is different from dried, unsintered gels, (typically produced by the sol-gel technique) in that the surface around the pores of heat treated/acid leached glass is still glass. In gels, the gelatinous material surrounding the pores is not a fully-formed glass, and, if improperly dehydroxylated, is susceptible to cracking when sintered. By contrast, porous silica glass is unlikely to encounter such problems, because the glass surrounding the pores is relatively strong. Due to these differences in properties, the dehydroxylation procedures for porous glass is not directly transferable to the treatment of dried gels.
In T. H. Elmer, "Nitride Glass," VII International Conference on Glass, vol. 1 (1965), porous silica glass is exposed to ammonia at elevated temperatures (i.e. 800.degree. C. or higher) to effect dehydroxylation according to the following reaction: EQU --Si--OH+NH.sub.3 .fwdarw.--Si=NH+H.sub.2 O
This technique, however, suffers from many of the same problems as those discussed above with respect to Susa.
Where the nitrogen-doped product resulting from such ammonia treatment is undesirable, that product may be subsequently contacted with heated chlorine to remove amine groups according to the following reaction scheme: EQU --Si--OH+--Si--OH.fwdarw.--Si--O--Si--+H.sub.2 O EQU H.sub.2 O+Cl.sub.2 .fwdarw.2HCl+O.sub.2 EQU --Si--OH+HCl.fwdarw.Si--Cl+H.sub.2 O EQU --Si=NH+--Si--Cl.fwdarw.N(Si--).sub.3 +HCl
See T. H. Elmer, "Chlorine Treatment of Nitrided Porous Glass," Glastech. Ber., 61, pp. 24-27 (1988).
It has also been known to use chlorine gas alone at elevated temperatures (i.e. above 700.degree. C.) to achieve dehydroxylation according to the following reaction scheme, as discussed in M. L. Hair et al., "Reaction of Chlorosilanes with Silica Surfaces," Journal of Physical Chemistry, vol. 73, no. 7, pp 2372-78 (1969): EQU 2(--Si--OH)+2Cl.sub.2 .fwdarw.2(--Si--Ci)+O.sub.2
Again, however, these techniques complicate the sintering step and require specialized gas-handling systems.
Dehydroxylation has also been carried out with carbon tetrachloride, as discussed in M. Shimizu et al., "Reaction of CCl.sub.4 with SiO.sub.2 Surfaces," Journal of the American Ceramics Society, vol. 54, pp. 271-72 (1971), and with chlorosilanes, as set forth in M. L. Hair et al., "Reaction of Chlorosilanes with Silica Surfaces," Journal of Physical Chemistry, vol. 73, no. 7, pp. 2372-78 (1969).
In both U.S. Pat. No. 4,772,305 to T. H. Elmer and T. H. Elmer "Dehydroxylation and Nitriding of Porous Glass by Means of Water-Soluble Nitrogen-Containing Organic Compounds," Glastech. Ber., vol. 60, pp. 399-405 (1987), an impregnation technique of dehydroxylation is proposed in place of the above-discussed gas treatment/sintering procedure. This impregnation process involves immersing porous silica glass in an aqueous solution of a nitrogen-containing organic compound, such as urea or a guanidine compound. The impregnated porous glass is then heated in a non-oxidizing atmosphere to dissociate nitrogen from the organic compound, combine the nitrogen with the glass, and expel hydroxyl groups. The glass may then be further heated to consolidate it to a non-porous condition. However, when utilized in conjunction with silica gels, this technique is not capable of producing a satisfactory glass product, because of cracking which occurs during sintering.